Disclaimer

I will use the color coding scheme which has become common among Pathfinder build handbooks. Also note that many colored items are also links to the Paizo SRD.

Red: Bad, useless options, or options which are extremely situational.

Orange: OK options, or useful options that only apply in rare circumstances

Green: Good options.

Blue: Fantastic options, often essential to the function of your character.

Temporary Note: Paizo has recently discontinued support of their official SRD. From this point forward SRD links on RPGBOT.net will instead link to d20PFSRD.com. While Archive of Nethys is now Paizo's officially licensed partner for the purposes of serving the Pathfinder SRD, Archive of Nethys is a horribly designed website and it simply doesn't match d20PFSRD's ability to keep pace with published content and d20PFSRD's search functionality.

If you encounter any links which still point to the old SRD, please email me so that I can correct them. I also recently added a page explaining my supported content which you may find helpful. --September 15, 2018

Introduction

Rules

Skill unlocks come into play every 5 skill ranks (5, 10, 15, 20), and provide gradually more powerful abilities beyond a linear improvement of your character's ability to use a skill successfully.

Who can use skill unlocks?

Your group might choose to allow all characters to use skill unlocks for all of their skills, which disproportionately rewards highly skilled characters. In groups which lean toward casters and other low-skill classes, this might be acceptable and might help balance out class usage.

You might instead require that characters take the Signature Skill feat, which allows characters to use skill unlocks for a single skill. You might also use the Unchained Rogue, which gives Rogues easy access to skill unlocks.

Personally, I recommend allowing both the Signature Skill feat and the Unchained Rogue. There is a huge variance in the quality of skill unlocks, and granting them for free to everyone disproportionately rewards a subset of characters who frequently take the skills with the best unlocks.

Skills

5 Ranks: Moving around in combat, especially by means of Acrobatics, isn't something which is done frequently. Pathfinder's combat is very static, and there are extremely few builds which break that norm. Those builds rarely lean on Acrobatics, and instead tend to focus on options like the Mobility feat or abilities which prevent attacks of opportunity without requiring a skill check.

10 Ranks: Very situational.

15 Ranks:Situational, but nice to have.

20 Ranks: At level 20 options for permanent flight are cheap and abundant. If you can't fly by now I'm not certain how you survived this far.

Appraise is a garbage skill, and skill unlocks do little to improve it.

5 Ranks:This can be done with Detect Magic and Identify, which are a cantrip and a 1st-level spell respectively.

10 Ranks: Ridiculously situational. I would allow a play to do this with an Appraise check without a skill unlock. I, a completely normal person with no special training in appraising things, could look at most people an make a reasonal guess which piece of a person's apparely is the most expensive piece. It's generally jewelry.

15 Ranks:Since you're probably not eyeing someone's jewelry in combat, reducing the action required is meaningless. The ability to appraise some illusions is weird, and very situational. The -10 penalty sounds like a lot, but skills scale much faster than saving throws, so your bonus may be decent the one or two times in your life that this comes up.

20 Ranks: Only useful if your character really likes to steal from people, and even then your best bet is probably Sleight of Hand.

Climb is a very situational skill, and can be easily replaced by a potion of Spider Climb for a meager 50 gp. At mid and high levels flight becomes prominent and important, which makes climb functionally useless. Skill unlocks only manage to emphasize this disparity.

Craft is for NPCs and poison enthusiasts. There are options to make magic item crafting available to non-casters using Craft, but they're all terrible compared to regular magic item crafting. Poison crafters might be able to make use of Craft's skill unlocks, especially since Rogues make up a disproportionate section of the poison-using population. Poison is very expensive, which means that it takes a long time to craft, and combining the 5-rank and 15-rank unlocks effectively multiplies your craft speed by 14. Combined with the existing rule which allows you to willingly increase your craft DC by 10, you can churn out poisons at a dizzying rate. Unfortunately, none of the unlocks do anything to address the problematic cost of poisons.

5 Ranks: Functionally doubles your craft speed.

10 Ranks: By this level it should be nearly impossible to fail a craft check.

15 Ranks: Multiply your crafting speed by 7.

20 Ranks: Blacksmithing, the most versatile craft skill in the game, now allows you to partially replace both Craft Magic Arms and Armor, Forge Ring, and big chunks of Craft Wondrous Item. Combined with the 5-rank and 15-rank abilities you can churn out magic items at a fair pace, possibly even faster than the normal 1000gp per day craft speed typical of magic item crafting.

Diplomacy enthusiasts like it because it's a great way to get out of a potentially lethal situation. The 10-rank and 15-rank abilities both play to this very well.

5 Ranks: This is generally something done in narrative time, so this will likely never matter.

10 Ranks: Influencing a creature in one round is a good way to diffuse a combat, and reducing the penalty to do so from -20 to -19 is pretty great. The improve duration for a shifted attitude will rarely matter.

15 Ranks: Removing the penalty entirely means that you can likely diffuse every combat with a sentient creature with which you share a language. The improve duration for a shifted attitude will rarely matter.

The skill unlocks for Disable Device pertain exclusively to traps, which makes sense, but they're really no good enough to make them interesting. The unlocks either allow you to disable traps faster or mitigate failure to disable a small subset of traps.

5 Ranks: Disarming a trap is typically done in narrative time, so this rarely matters.

10 Ranks: If you don't have Trapfinding but you're taking Disable Device ranks, you are wasting your skill ranks. If you do have Trapfinding, you shouldn't plan to fail checks to disable traps. If you fail, suffer the consequences of failure and get on with your life.

15 Ranks: This is both dumb and bad. How exactly does a Disable Device check protect you from things like arrows or darts?

20 Ranks: The ability to disable a trap as an immediate action means that the most time-effective way to disable traps which make attack rolls is to intentionally trigger them. This requires the 15-rank unlock, so Rogues will likely want to skip this rather than spend two of their four free unlocks for a single situational ability.

Despite the importance of flight at high levels, Fly is a nearly worthless skill. You get a +8 bonus just for having a fly speed, so one rank and the class skill bonus gives you a comfortable +12 bonus (before your ability score), which will get you through most effects in the game which call for Fly checks. The Fly skill unlocks make you more able to change direction in the air, and allow you better resist pesky wind effects, so in games with a lot of flight and counter-flight effects, they can be helpful.

Handle Animal is used almost exclusively for Animal Companions, and the skill unlocks reflect that. Druids and Hunters are the only ones who will get any real use out of Handle Animal.

5 Ranks: Nice, especially if your companion is a mount.

10 Ranks: Animals have poor will saves, and this matches the effects of Iron Will if you're in range.

15 Ranks: The ability to speak to your animal companion without Speak With Animals can greatly improve the utility of your animal companion depending on how your group runs pets. Training typically happens off-screen during narrative time, so doing it faster rarely matters.

20 Ranks: By this level Druids and Hunters can cast Speak With Animals easily.

In 3.5 Heal was a joke skill on par with Use Rope. You took it to qualify for shitty feats and prestige classes. In Pathfinder the "Treat Deadly Wounds" option makes Heal a fantastic and cost-effective way to boost your party's healing abilities. Heal's skill unlocks further improve the Treat Deadly Wounds to provide additional healing. The max DC you need to hit is 25, which is fairly easy with a few ranks, decent Wisdom, and items. Beyond that, skill unlocks are the only reason to continue investing ranks. Each unlock provides a better version of the same effect, so I believe the intent is that they do not stack, though the rules don't say so specifically.

5 Ranks: 2 hp per hit die, and 2 ability damage off of each damaged ability. At this level that's a huge chunk of healing, and will save precious spell slots and/or costly wand charges.

10 Ranks: 4 hp per hit die, and 4 ability damage off of each damaged ability.

Intimidate has two uses: Intimidating people in conversation, and fear-stacking. Using Intimidate to demoralize a foe is standard action, which makes it hard to justify in most fights, but many abilities allow you to use it for free.

5 Ranks: Beating the Intimidate DC by 10 will be hard, especially at low levels, but 1 round of making an enemy frightened will keep them from attacking you for a round. Fear stacking builds may be able to repeat this effect several times,

15 Ranks: Beating the Intimidate DC by 20 is hard, but with a bit of optimization (items, skill focus, etc.) you may be able to do it reasonably often. Cowering foes are considered helpless, which means that they can be killed with a coup de grace. Unfortunately the 1 round duration means that the effect ends at the start of your next turn, so you will need help from an ally.

20 Ranks: Cowering for 1d4 rounds means that three fourths of the time you can coup de grace targets all by yourself.

One of the most important uses of Knowledge skills is the ability to identify enemies, and the Knowledge skill unlocks play to this very nicely. Unfortunately, selecting Knowledge with Signature Skill only applies to a single knowledge skill, and unless you face that creature type almost exclusively you're not going to get a lot of use out of your Knowledge unlocks.

5 Ranks: It's hard to rate this one because there is so little concrete guidance on what information to give when a player makes a knowledge check to identify a crature.

10 Ranks: This looks a lot like Knowledge Devotion from 3.5, which was a great feat for intelligent martial characters like the Duskblade, and works great in Pathfinder for the Magus. Unfortunately, since Signature Skill and Rogue's Edge only work on a single skill, this is really only useful if your group allows everyone to use skill unlocks for free.

15 Ranks: Knowledge is very important, and failing to identify a monster can be the difference between life and death. The bonus to saves is also nice, especially since it increase to +2 at 15 ranks.

20 Ranks: Remember that this only applies to the specific knowledge skill in which you have 20 ranks and take Signature Skill or for which you use your Rogue's Edge.

Linguistics is mildly amusing until you can cast Tongues, at which point it goes the way of Use Rope. The skill unlocks are terribly and not particularly well worded.

5 Ranks: Very situational.

10 Ranks: What the hell? This bit doesn't specify that the writing/speech needs to be one including a coded message, which means you need to make a Linguistics check to understand perfectly mudane speech. Some guy could say "I enjoy hard-boiled eggs" and unless you passed a DC 30 Linguistics check you have no idea what he means.

Like Perform itself, Perform's skill unlocks are really only viable and useful for Bards. That said, Bards will get a lot of use out of these effects.

5 Ranks: Once you're good enough at Performn this turns into a free +2 bonus to 4 skills, 3 of which are important for any Face.

10 Ranks: Nice for Bards, but situational.

15 Ranks: Nice for Bards, but situational.

20 Ranks: Bards will be using their Versatile Performance ability to use their Perform modifier in place of Diplomacy, so you're probably going to be making two Perform checks and choosing the better result.

Sense Motive's skill unlocks are surprisingly good. The initiative bonus is great for Sneak Attack users and for spellcasters, and the 15-rank and 20-rank unlocks provide some excellent defensive options.

5 Ranks: Initiative checks are very important, and both Clerics and Rogues can benefit greatly from this ability.

10 Ranks: a -20 penalty is very difficult to overcome.

15 Ranks: There doesn't appear to be a limit on how often you can make use of the AC boost, so you can use this for a frequent +2 insight bonus to AC.

Sleight of Hand is a Rogue skill which I frequently skip over because it sees so little use. Even stealing items can be done as a Combat Maneuver, making Sleight of Hand that much less useful. The first unlock allows you to combine a Steal maneuver with a Sleight of Hand check for a paltry bonus, and the rest of the unlocks all pertain to quickly drawing hidden weapons, which only a handful of Rogues will ever care to do.

5 Ranks: I hate that there is both the Steal maneuver and the Sleight of Hand check to steal things from people. Oh look, now you can use both at the same time. Approved by the Department of Redundancy Department.

10 Ranks: Helpful, especially for Rogues who like to use hidden weapons.

15 Ranks: Situational, but it allows you to draw a hidden weapon and make a full attack in the same round.

20 Ranks: Dramatically improves the speed at which you can reliably access hidden weapons.

5 Ranks: Both of these actions take place in narrative time, so this will almost never matter.

10 Ranks: Detect magic is a cantrip, and if you don't have it you have made a poor life choice.

15 Ranks: These actions still take place in narrative time.

20 Ranks: You will need a totaly +40 Spellcraft bonus before you have the same chance of succeeding on this Spellcraft check as you have to pass the unboosted caster level check. This ability is awful.